Prior art high power RF power transistors utilize a ceramic substrate, typically made from Beryllium Oxide (BeO), onto which a die or an array of dies forming the circuitry of the device is affixed. The BeO substrate is mounted on expensive Copper-Tungsten (CuW) base which can then be mounted onto a heat sink. CuW is used for the base because of its high thermal performance having the ability to efficiently conduct heat from the BeO base to the heat sink and has the same coefficient of thermal expansion (CTE) as the BeO substrate and silicon die. This comparable CTE minimizes fatigue of the joining materials used at the different interfaces.
An example of a prior art RF power transistor package 100 using a ceramic substrate 102 mounted on a CuW base 104 is shown in FIGS. 1A and 1B. The cover 106 overlies the ceramic substrate 104. To ensure an appropriate thermal path between the ceramic substrate 102 and the CuW base 104, the bottom of the ceramic base 102 must be metallized and then brazed to the CuW base 104. This example of an RF package 100 is an MRF154 RF MOSFET manufactured by M/A-COM.
The package 100 includes a drain lead 112 and a gate lead 114. The electrically conductive CuW base 104 also serves as the source lead for the RF package device 100. An insulator 102 separates the drain and gate leads 112, 114 from the CuW base 104, a shown in the side elevation view in FIG. 11B.
The CuW base 104, however, has a CTE different from that of the heat sink 108, which typically comprises Copper (Cu) or Aluminum (Al). The heat sink 108 expands and contracts more than the CuW base 104 as the device in package 100 controls power in cycles giving rise to numerous heat cycles seen in normal operation. As the heat sink 108 cools and contracts, the fasteners 110 mounting the CuW base 104 through holes 118 to the heat sink 108 constrain both materials from expanding and contracting freely to their natural extent and will deform the CuW base 104, causing the CuW base 104 to bow up and away from the heat sink 108, creating a gap between the CuW base 104 and the heat sink as time progresses. This bowing decreases the thermal performance of the CuW base 104 because less surface area of the bottom of the CuW base 104 is in contact with the heat sink 108. The bowing also causes the ceramic substrate 102 to separate from the CuW base 104, further reducing the thermal effectiveness of the CuW base to transfer heat from the ceramic substrate 102 to the heat sink 108, thereby decreasing the effective life of the package 100.
An example of another prior art RF power transistor package 120 is shown in FIG. 1C. In this package 120, the CuW base is eliminated, with the ceramic substrate 122 also serving as the base for the package 120. The thermal path from the heat sink 128 to the ceramic substrate/base 122 is now direct. The clamp 126 overlying the cover 124 clamps the ceramic substrate/base 122 tightly against the heat sink 128 with fasteners 130. An example of such a package is an ARF 1500 RF Power MOSFET manufactured by Advanced Power Technology.
The clamp 126 may be made from any suitable strong material such as steel or aluminum. Clamp 126 holds the substrate/base 122 in tight contact with the heat sink 128 from above. In this construction, the substrate/base 122 is directly fastened together with the heat sink 128. The differential expansion rates between the ceramic base 122, typically made of BeO, and the heat sink causes the ceramic base 122 to polish or lap the interface surface and improve the thermal transfer between the base 122 and heat sink 128 over continued thermal cycles.
What is needed is a power resistor or transistor package that utilizes a ceramic substrate as a base that is kept in proper contact with a heat sink over the normal expected life of the device without using extra, separate clamping devices.